Ports `Variant` (cs:1170-1241), `AsbStatus` (cs:1109-1167), `RuntimeValue`
(cs:741-791), `AsbVariantFactory.From*` (cs:1310-1429), and
`MxAsbDataClient.DecodeVariant` (cs:713-825) into `mxaccess-codec::asb_variant`.
Three layers per `docs/ASB-Variant-Wire-Format.md`:
1. `AsbVariant` — raw 2/4/4/payload header + bytes; round-trips byte-identical.
2. `DecodedVariant` — typed view with one variant per proven ASB scalar / array
(`Bool`, `Int32`, `Float`, `Double`, `String`, `DateTime`, `Duration` plus
array forms). Type ids outside the proven matrix surface as
`Unsupported { type_id, payload }` — same fallback as .NET's `_ => payload`.
3. `from_*` factories — mirror `AsbVariantFactory.FromX` exactly, setting
`length` to `payload.len()` per `cs:1431-1438`.
`AsbStatus` and `RuntimeValue` round-trip the wire layout verbatim.
Status-element walking (marker bit 7 = implicit zero, etc., per
`docs/ASB-Variant-Wire-Format.md:180-205`) is deferred to a follow-up; the
codec exposes the raw status payload bytes for now, matching .NET's
`AsbStatus.Payload = byte[]` shape.
The lib.rs `AsbVariant` / `AsbStatus` / `RuntimeValue` stubs are replaced by
the real types via `pub use`. 25 new unit tests cover the proven matrix:
scalar + array round-trip, byte layout (2/4/4/payload), `Unsupported`
fallback for declared-but-unproven types, short-frame rejection,
malformed `string[]` partial-decode preservation matching .NET behavior.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
7611d9e215